Borane-Catalyzed C–F Bond Functionalization of gem-Difluorocyclopropenes Enables the Synthesis of Orphaned Cyclopropanes

Herein, we disclose an approach to synthesize tert-alkyl cyclopropanes by leveraging C–F bond functionalization of gem-difluorocyclopropenes using tris­(pentafluorophenyl)­borane catalysis. The reaction proceeds through the intermediacy of a fluorocyclopropenium ion, which was confirmed by the isolation of [Ph2(C6D5)­C3]+[(C6F5)3BF]−. We found that silylketene acetal nucleophiles were optimal reaction partners with fluorocyclopropenium ion intermediates yielding fully substituted cyclopropenes functionalized with two α-tert-alkyl centers (63–93% yield). The regioselectivity of the addition to cyclopropenium ions is controlled by their steric and electronic properties and enables access to 3,3-bis­(difluoromethyl)­cyclopropenes in short order. The resulting cyclopropene products are readily reduced to the corresponding orphaned cyclopropanes under hydrogenation conditions. Quantum chemical calculations reveal the nature of the C–F bond cleavage steps and provide evidence for catalysis by boron and not silylated oxonium ions, though Si–F bond formation is the enthalpic driving force for the reaction.